In carpal tunnel syndrome, which motor function is typically weakened first?

• A. Interossei function
• B. Hypothenar eminence function
• C. Thenar eminence function (thumb opposition/abduction)
• D. Wrist flexors strength

Answer: (C)

Compression of the median nerve at the wrist most affects the motor fibers to the thenar muscles via the recurrent branch. Those muscles—primarily responsible for thumb opposition and abduction—weaken first, so you’ll see difficulty opposing the thumb to the fingers and a reduced ability to pinch or grip. The other hand muscles mentioned are innervated by the ulnar nerve (interossei and hypothenar) or are forearm muscles supplied higher up the nerve, so their weakness isn’t the earliest sign in carpal tunnel syndrome.